Damselfish, any of about 250 species of small, primarily tropical marine fishes of the family Pomacentride ( order Perciformes) found in the Atlantic and Indo-Pacific oceans. Damselfishes are deep-bodied and usually have forked tails. They resemble the related cichlids and, like the, have a single nostril on each side of the head and have interrupted lateral lines. Damselfishes have two anal spines. Many species are brilliantly colored, often in shades of red, orange, yellow, or blue; most do not exceed a length of about 15 cm (6 inches).

Damselfishes are lively and quick, and are usually strongly territorial and aggressive. Some feed mainly on plant matter or small animals suspended in the water; others are omnivorous. Most damselfishes live along reefs, but certain species, the anemone fishes, are noted for living among the stinging tentacles of sea anemones.